I actually started the TS location with the Na+ in the middle, but it gradually moved to one side, and so the system ended up unsymmetrical. I guess it gets more dipole stabilisation there than in the middle (much like the carbon in a sense also prefers an unsymmetrical equilibrium position). In this regard, it is simple to simple MO perturbation theory (the Kloopman-Salem equation), where one large and one small pairwise overlap is generally more stable than two medium overlaps.

A better model would be to include say another NaBr molecule, and perhaps a few explicit solvent molecules. Ionic reactions are certainly ones where the density of states is high; and many (almost equivalent) arrangements of the atoms are possible. So a true analysis would have to be a statistical one. But in fact, removing the Na, and computing a system with an overall charge of -1 reveals almost exactly the same behaviour, so in this case, the exact location of the counter-ion may not be crucial.

The fact that the IRC is not symmetrical may seem confusing. Do you think it is because of the Na+ position (being closer to one bromine than to the other)?

Nicolas